El Salvador is about 3.6 times smaller than Panama.

Panama is approximately 75,420 sq km, while El Salvador is approximately 21,041 sq km, making El Salvador 27.9% the size of Panama. Meanwhile, the population of Panama is ~4.3 million people (2.2 million more people live in El Salvador).
Tip: This to-scale comparison of Panama vs. El Salvador uses the Mercator projection, which distorts the size of regions near the poles.